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  • The ignition control module (ICM) switches the ignition coils on and off to fire the spark plugs in older vehicles. It determines your vehicle’s base ignition timing and controls the primary ignition circuit to aid in ignition.
  • A bad ICM can lead to an illuminated check engine light, engine misfires or a rough engine, stalling, or your vehicle not starting.
  • Before replacing your ICM, run a test to avoid misdiagnoses. A replacement ignition control module can cost between $50 and $250.

Each of the components of the ignition system plays a vital role in producing the voltage needed by the spark plugs for combustion. In this article, we’ll discuss one of the components that’s a major part of many ignition systems—the ignition control module (ICM).

What Is an Ignition Control Module?

Many older vehicles use a stand-alone ignition control module. It is responsible for switching the ignition coil(s) on and off to fire the spark plugs. This is crucial, as the engine can’t run properly unless every spark plug is fired at exactly the right time.

What Does the Ignition Control Module Do?

The ignition control module (ICM) gathers information from a triggering device (usually the crankshaft position sensor or camshaft position sensor) to determine your vehicle’s base ignition timing. Correct ignition timing is crucial because it plays a major role in your engine’s performance and health.

MSD Ignition Control Module
Many older vehicles use a stand-alone ignition control module. It is responsible for switching the ignition coil(s) on and off to fire the spark plugs.

The intricacies of your ignition system may vary depending on the specific vehicle you own. On most modern models, the powertrain control module receives input from sensors and controls the operation of your ICM. On some vehicles, the ICM only controls ignition timing on lower RPMs. To get a better understanding of what the ignition control module does, you must know how the ignition system works.

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The Ignition System

The ignition system is composed of parts and wirings that work together to create and distribute the voltage power needed by your spark plugs. This process creates the spark that raises the temperature in the air-fuel mixture, making it possible to start combustion.

The ignition coil is the heart of your vehicle’s ignition system. It usually contains two separate windings of copper wires. In a traditional ignition system with a single coil, the ignition switch is attached to the positive terminal, where the current from the positive battery terminal is supplied. On the other hand, the negative terminal is attached to the ICM, which opens and closes the primary ignition circuit to fire the spark plugs via the distributor.

Bad Ignition Control Module Symptoms

Because the ICM is an integral part of many ignition systems, it must be replaced as soon as it starts failing. Here are the common symptoms of a faulty ICM:

Illuminated Check Engine Light

Your check engine light will illuminate once it detects something wrong with your ignition system. If you have an OBD scan tool, you can insert it into your ride’s OBD port to retrieve the error code. Diagnostic codes P0300 to P0399 are examples of ignition-related error codes. If you don’t have the proper diagnostic tools, you can always take your vehicle to an auto repair shop for inspection.

Engine Misfires and/or Rough Engine

An engine misfire is usually a symptom of incomplete combustion. The same goes for a rough-running engine. You may also have issues accelerating if your ICM is faulty.


A failing ICM may cause stalling because it hinders the engine from getting the spark it needs. However, take note that stalling may also be caused by other faulty engine components. So it’s best to take your vehicle to an auto repair shop for proper diagnosis.

Vehicle Won’t Start

A no-start condition usually happens when the ICM has failed. To keep your vehicle in top shape, have it inspected right away once you experience any of the symptoms mentioned above.

Think twice about driving with a bad ICM, as it will eventually damage your catalytic converter, which can result in more problems.

Ignition Control Module Test

Don’t replace your ignition module right off the bat. Conduct proper testing to avoid wasting your time and money on a replacement that you may not need.

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mechanic performs work on replacing the ignition module
Conduct proper testing to avoid wasting your time and money on a replacement that you may not need.

There are many articles on how to test the ignition control module online. You can use them if you’d like to diagnose the problem yourself. You’ll need a spark tester, test light, and an ignition control module tester to get started. However, take note that testing the ICM  is not a simple process, so it’s best to leave the job to professionals, especially if you’re not an experienced DIYer.

Common Misdiagnoses

Other faulty ignition-related components may also cause engine misfires. Spark plugs, spark plug wires, and ignition coils are some of the parts that are usually replaced to fix misfires before testing the ICM.

How Much Does an Ignition Control Module Replacement Cost?

A replacement ignition control module can cost anywhere between $50 and $250. Labor costs can range anywhere between $70 and $90. Remember that these are only rough estimates, and you’ll have to factor in expenses for other related repairs.

How to Fix an Ignition Control Module

The difficulty level of ICM repair depends on your vehicle. That’s because the ignition control module location may vary per vehicle manufacturer. Accessing the ICM can be fairly easy in some, while it takes more work to access others.

If you suspect that your ICM is faulty, have your vehicle inspected by a mechanic right away. Have any faulty components replaced, so you can restore your ride’s performance in no time.

A Closer Look at the Ignition Control Module

Each spark plug has a center electrode and a ground electrode, and the energy that jumps the gap between those two electrodes to ignite the air/fuel mix is an amplified voltage that can be over 100,000 volts on modern vehicles. This spark must happen at the right time, but that right time changes depending on engine speed and load.

The ignition coil is a transformer that amplifies the vehicle’s 12-14 volts for this purpose. The coil is triggered either by a stand-alone ignition module or by circuits within the ECM/PCM itself (which is more common on late model vehicles).

How Ignition Modules Came About

Note: For more information on the development of ignition systems, you can read our discussion here.

GM toyed with electronic ignition systems (using a transistor in a module rather than contact points) as far back as the late 1940s. But ignition systems widely began to evolve. Chrysler first made electronic ignition the standard on all its vehicles in 1972, with a module triggering their oil-filled coil based on input from a wire-wound stator magnet mounted next to a spinning reluctor within the distributor. But the distributor still had a vacuum advance and an internal centrifugal advance for on-the-fly timing changes.

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Beginning in the mid-70s with GM’s HEI ignition, oil filled coils began to disappear and were replaced by potted E-core ignition coils that worked the same way the oil filled coils did, except that the voltage increased from about 50,000 volts to nearly 100,000 volts.

Ignition modules using transistors were the order of things by this time, and as computers began to appear on vehicles, the ignition module’s timing of the ignition event began to be modified by the ECM.

In the mid-80s, distributors began to disappear, replaced by coil packs.

But ignition modules were still the box that fired the coils, albeit with ECM modifying the module’s timing of ignition pulses.

Later, the ECM/PCM began to receive the CKP signal and control the coil primary triggers, so that the ignition module became a part of the ECM/PCM.

How to Get a Reliable Ignition Control Module for Your Vehicle Online

A bad ignition control module can cause engine problems like misfires, rough running, or stalling. If you ignore its symptoms, you might find it impossible to start your vehicle one day. When you need a replacement ignition control module that you can rely on, look no further than lets you order a new ignition control module online from the comfort of your home or office. Visit our easy-to-use website and search our catalog of high-quality aftermarket parts and accessories by entering your vehicle’s details into our integrated vehicle selector. Placing an order takes only a few clicks and our secure checkout process gives you the safe online shopping experience you deserve.

Why delay replacing an ignition control module that has seen better days? Take a look at our wide selection of ignition control modules here at and shop now.

About The Authors
Written By Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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Darryl Hicks

do you need to retime engine after repair or replace og module

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